1) How do you go from needing 30kw to drive at 6kn to 6kw to drive at 6kn? Shaft hp requirements are the same regardless of the type of propulsionmotor. In fact the type of propulsion is immaterial to the power required to turn a shaft at a given speed.

If a boat requires 30kw of shaft power to move at 6kn it will always require 30kw of shaft hp to move at 6kn. No change to the propulsion device effects this.

2) As discussed above, which you refused to appreciate. In order to get more thrust at a lower rpm you have to use a MUCH larger prop. The only thing that changing to a larger prop does is effect the slippage rate, it has nothing to do with overcoming the force of the wind. So if the wind is applying a force equal to 20kw of power in one direction it will take a static force of 20kw of force applied in the opposite to keep you still. It could be a small prop spinning quickly, or a large prop spinning slowely but it takes the same amount of input force.

3) if you take a smaller prop and spin it at the same speed as a large prop of course the larger prop will generate more power. However if you take a small prop and apply 10hp to turning it, and you take a large prop and apply 10hp to turning it the output power will be pretty close to the same (the difference is in prop efficiency, which for reasonable options is minimal).

And keep in mind almost every sailboat made uses the largest prop possible given shaft location. To increase to a prop large enough to make much of a difference you have to redesign the boat.

Finally: you are wrong. Electric power does have some advantages. The prototypical case is a boat that is only day sailed, has a short distance to travel in and out of the harbor, and then returns to the harbor to be recharged at night. The further you go from this ideal the worse the trade offs are. For a cruising boat they are immense, which is why there are almost no cruisers using electric propulsion, and why every major manufacturer has backed out of projects to deliver them.

Thanks Greg. But on your first point 1) above, where did I ever say it took 30kW to go 6 knots? I never said or implied that. I said the motors are 30kW motors, that is their input spec. I do not know what the hull speed will be at 30kW ie full power. I said that it has been estimated the hull speed would be about 6 knots running the 30kW motors at 6kW. Chris below in his latest post says this is fanciful, and he may be right but I'll explain why we came up with this figure in the response to his post.

2) OK, I hear what you are saying. I will go and try and clarify this and get the specific numbers before I can confirm or deny.

3) This is news to me that if you apply the same force (you mean torque?) to a small prop and large prop you get the same output force (you mean thrust?) That is counter-intuitive to me, but OK. Can't see why anyone would want a bigger prop then, might as well save the money. Then you go on to say that pretty much everyone puts on the biggest prop possible?

And then in your last paragraph you tell me I'm wrong (wrong about what, exactly?), that EP is only for puttering in & out of harbour, and that for cruising the trade-offs (what are they specifically?) are immense, so clearly it won't work. But can you see my issue with this sort of answer? They're just statements of opinion, with no justification. That is what I'm calling as FUD. Be specific. Why do you believe that? If you have good reasons, please share them.

Then you justify your opinion by the authority of major manufacturers who have "backed out" of EP projects. Well, I know about the Lagoon mess, that's one (1), that self-immolated by screwing up the system as it was designed for them. Serves them right. But that is not reason to say EP per se, is not achieveable, is it?. Besides Lagoon, what other major manufacturers are you aware of?

Upwind heavy seas requires marked increase in power not available within your strategies.
Bottom Line is solar conversion efficiencies and/or KwH batt power densities per liter are just NOT there to compete with diesel on many levels yet.
This movie in head.

BTW,Calder is no engineer nor navel architect.We've all read and continue to read what's there and coming and look forward to the day.

No you are not. You refuse to acknowledge the reality of people who live in the real world and cruise every day.

By the time your 46" cat is loaded up with all of this stuff it will be about the same weight as any other similar boat of the same size.

My boat for safe cruising in any number of circumstances requires 30Kw PER MOTOR at the prop. Yes, lighter and faster boats may get away with down to 20Kw, but 6Kw is simply fanciful.

Kw at the prop is the same for electric and diesel as Greg notes. This is a matter of simple physics.

On a 46" cat when you are motoring into a 40kn blow on the nose and a 3m swell you are going to need both motors for at least 60Kw to have a margin of safety.

So I put this proposition to you. Go out on a cat of this length, and confine the revs on a single motor to 6Kw as per the torque curve and see how you go. That is if you can find someone who will do this as it would be unsafe in the extreme.

Chris,

Yes I am. I am open to being convinced by evidence, in every aspect of my life that I can think of, and this is no exception. I've been trained well in science, and when better data is presented, it changes minds. Mine included.

Now, in this post of yours there is some good info on what you think your boat needs in kW at the prop. That is helpful, so thanks

And in Greg's post he is making the case that bigger higher pitched props will not give much more thrust. That's not my understanding from my reading, but I will revisit that area. My understanding is that when you need to push a boat against big forces, like an ice breaker does, you use big high pitched props turned at low revs by electric motors, and that generates alot of thrust. As I understand it electric is used because of the torque characteristics of electric, because diesel engine driven big props will overload and/or stall the motor. Anything about that that I've got wrong?

I'll explain a bit about where the 6 knots at 6kW input comes from. To be clear, this is for cruising along motoring, not for adverse conditions or emergency, right? Understood?

When you say you need 30kW PER MOTOR for any number of circumstances, are those adverse conditions like high wind, tidal currents,etc. or just motoring along? If it is for adverse conditions, then why compare it to my 6kW for just pottering along in calm conditions and call it fanciful?

Obviously, if there are adverse conditions,we can push the throttle down too.

Now, to the 6 knots with 6kW input power to the 30kW motors. Without doing tank testing, which actually may happen BTW, we extrapolate the real world results of looking at performance of electric motors on known boats. Here is one example of an Oram 44 that has been using 2 x Torqeedo Cruise 4.0, (a 4 kW motor) for the past 5 years. The owner reports, and I have talked to someone else who's evaluated Cruise 4 on this boat, that he gets 4.5 to 5 knots running the motors at 2 kW, ie half throttle. We know what the output thrust is exactly in this example and so we can do a "guesstimate" on the 30kW motors using 6kW by taking the comparative thrust data. We also know that the hull that the electric is for was driven at about 7 knots at idle in smooth water with Yanmar 39HP with known Brunton Autoprops that were over pitched by a known amount, ie we know that thrust. So we're not really trying to be fanciful, we're actually pretty serious about this. Of course, we could be totally wrong, as you and Greg would say, and time will tell the tale as they say.

So in your eyes it may be fanciful, but actually I think in your haste to shoot down any discussion of EP, you compared a motoring in becalmed condition with EP at 6kW to your Helia situation at 30kW PER MOTOR in more adverse conditions. Not an equivalent comparison.

As to your scenario of 40 knots on the nose in 3 meter swell, I have hand steered a 47' cat in 40 knots gusting 55 fore reaching with a storm sail into a big breaking sea (much bigger than 3 meters that's for sure) for 2 days (there were 2 of us alternating. Still exhausting!), so next time I'm faced with that I'll be on the parachute anchor, not motoring thank you very much.

I am not sure of Keshi's power use at 6klm but she was a very fast hull under power and at idle so I would imagine its very low.

Nathan indicated a top speed under power of 12 knots with clean hull although I cannot confirm that personally.

Hiya John

Greg said they had 7 & 8 knots AT IDLE in the marina and it scared the hell out of him. That was the slowest he could go & had to pop it into and out of gear. The Bruntons were overpitched, so when he changed the props it became less scary in the marina. With the props he has now he confirmed he gets 12 to 13 knots in smooth water. When I was there we got 10 but he had enough weed on the hulls to start a kelp farm. Still pretty awesome Those hulls are SLIPPERY!

What are your thoughts on all of this EP stuff? Am I delusional as most of the posters would think?

Upwind heavy seas requires marked increase in power not available within your strategies.
Bottom Line is solar conversion efficiencies and/or KwH batt power densities per liter are just NOT there to compete with diesel on many levels yet.
This movie in head.

BTW,Calder is no engineer nor navel architect.We've all read and continue to read what's there and coming and look forward to the day.

All the Best

Not sure where I have posted incorrect conversions, I know the kW to HP conversion.

I may have talked about the motors in kW and HP and typed the wrong unit of measure somewhere, but they are 30kW = 40 HP.

Could you expand on your heavy upwind seas comment, ie what scenarios are you thinking of? Motoring up a river with big headwinds, that sort of thing?

If we had to motor longer than an hour on full throttle, yep, that would be an issue. But refer to my comments on whether full throttle would actually be needed. What I would like to find is a wind chart that shows the force on a catamaran at increasing wind speeds. That would be useful.

Calder may not have formal qualifications as an engineer but it sure seems he understands the principles and can apply them. Is there anyone else that is writing about this subject from an experience base? Please refer me to the info if you've found it, because i have searched.

I have heard that comment about energy densities/efficienies many times, and have thought about it alot. I still am wondering why EP has to be exactly like diesel in order to be valid. There are differences, I get that. If you want to motor at full speed for days on end, yep, diesel is for you, no doubt about it. But otherwise???

For you, in what cruising scenario is the claimed limitation of EP most obvious?

That is more info than what I had but 12 knots shows its a very fast slippery hull.

On EP I am yet to be convinced away from diesel, but sitting back and observing the discussion. The hulls have the capacity to carry 1200l underfloor which should give enough capacity when in remote areas.

I remember having these futile discussions with the first owner of the Corsair 50 who swore that EP motors would be the miracle solution. He was as dogmatic as you in his thinking and would not see reason.

Sure enough the boat went into the water and it frightened the crap out of him, and the boat was lifted and diesels fitted.

But it has been my experience that arguing with idealogues is like hitting ones head against the wall. Whatever anybody says they will not be convinced until such time as they learn the hard way.

Having 5 kids I have learned this is part of the human condition, and people who listen to argument based on science are in the minority.

I have no more to say on the issue. You will go ahead and do this anyway, so I look forward to hearing the outcome.

I remember having these futile discussions with the first owner of the Corsair 50 who swore that EP motors would be the miracle solution. He was as dogmatic as you in his thinking and would not see reason.

Sure enough the boat went into the water and it frightened the crap out of him, and the boat was lifted and diesels fitted.

But it has been my experience that arguing with idealogues is like hitting ones head against the wall. Whatever anybody says they will not be convinced until such time as they learn the hard way.

Having 5 kids I have learned this is part of the human condition, and people who listen to argument based on science are in the minority.

I have no more to say on the issue. You will go ahead and do this anyway, so I look forward to hearing the outcome.

Chris, really, mate

You just seem to be the kind of guy who likes to portray they know alot about a subject when they really don't, and then get's huffy when anyone questions your statements. You have made statements that amply demonstrate you don't know as much about EP as you think you do, and you obviously don't know much at all about the specific system I am describing, and yet you've assumed all sorts of information that is just dead wrong like

it would take 400kWh to motor at 2 knots for 24 hrs in Post #34,

that the propulsion batteries would be 1 kW (which would be absurd BTW) in Post #83,

then when I corrected you, showed amazement that they were 13kW and that this all contravened the laws of physics Post #86

And along the way I'm "risking my loved ones" Post #58, I'm "fanciful", "closed minded and won't listen to experienced cruisers" ( that one is way off base), and now I'm an "ideologue".

OK, whatever. I've argued my case re EP, and I'm sorry that offends you, that I just don't throw up my hands in the revelation of your wisdom and tell Torqeedo with their multi million dollar R&D program that they are on a fools mission. Clearly you know better than they.

Now, to the 6 knots with 6kW input power to the 30kW motors. Without doing tank testing, which actually may happen BTW, we extrapolate the real world results of looking at performance of electric motors on known boats. Here is one example of an Oram 44 that has been using 2 x Torqeedo Cruise 4.0, (a 4 kW motor) for the past 5 years. The owner reports, and I have talked to someone else who's evaluated Cruise 4 on this boat, that he gets 4.5 to 5 knots running the motors at 2 kW, ie half throttle. We know what the output thrust is exactly in this example and so we can do a "guesstimate" on the 30kW motors using 6kW by taking the comparative thrust data.

What was the wind and tide in these scenarios? TBH, based on my recent experience I'd be very wary of just taking what Neil says at face value. I have no doubt he has a financial incentive to sell Torqueedo's.

From memory, School's Out needed one motor running at near full power to achieve 5 knots in calm conditions. Both motors flat out reached about 7.5 knots SOG, but there was certainly some tidal assistance in that figure.

And bear in mind that the Torqueedo props are optimised for forward thrust. Reverse thrust is very poor. Virtually non-existant.

In my Oram 44C it takes much more than 2 kW to do 5 knots. I'd estimate more like 6 or 7 Kw, admittedly with props which work equally well in reverse.

I'd suggest that for a 6 tonne, 44 foot cat, you should budget on more like 10 kW to motor at 6 knots in calm conditions.

What was the wind and tide in these scenarios? TBH, based on my recent experience I'd be very wary of just taking what Neil says at face value. I have no doubt he has a financial incentive to sell Torqueedo's.

From memory, School's Out needed one motor running at near full power to achieve 5 knots in calm conditions. Both motors flat out reached about 7.5 knots SOG, but there was certainly some tidal assistance in that figure.

And bear in mind that the Torqueedo props are optimised for forward thrust. Reverse thrust is very poor. Virtually non-existant.

In my Oram 44C it takes much more than 2 kW to do 5 knots. I'd estimate more like 6 or 7 Kw, admittedly with props which work equally well in reverse.

I'd suggest that for a 6 tonne, 44 foot cat, you should budget on more like 10 kW to motor at 6 knots in calm conditions.

Thanks 44CC,

Yeah, really until either the proper tank testing or put the real boat in the water and test it happens, it is guesstimating , as I've said. It would be nice to know the margin of error plus or minus a couple of knots, so that's what I'm aiming for.

The new Cruise 4's have several improvements, although I think the prop was improved substantially quite awhile ago re reverse.

BTW, what do you think of using the Cruise 4 for the tender? The reason I raise it was I spoke to a guy cruising the GBR and parts north, and he said they went through so much more petrol for their 9,9 Yammie than they thought they would, in fact they ran out and had to head somewhere to re-fill the jerrycans. They were doing alot of diving and fishing everyday, so used the tender alot.

Was thinking the tender sunshade roof could be all semi rigid solar panels to keep the electric outboardbattery topped up? You wouldn't go as fast as with a petrol outboard, but you'd keep going.

I have checked as many vendors as I can, and Torqeedo is the most efficient motor (54%, input kW to output kW) that I could find and they publish the figures, unlike other vendors who do not publish any tested efficiency data.

The 54% efficiency is when pushing a lightweight skiff slowly. The efficiency drops a lot when pushing a larger boat without changing the propeller.

Quote:

Originally Posted by Stumble

Sure they can charge off solar. But figure out the size and cost of the solar system you need in order to drive one. Let's assume a relatively modest 4kw/hr draw from the system over the course of the day. When you spec out the size of the panels you need to power this you will be suprized how big a boat you will need to build just to mount it.

Exactly. They can run on solar, but the torqeedo motors are not designed or intended to use solar as the primary source of charge. For this reason, their products tend to be drop-in replacements for existing fossil fuel motors rather than taking the needed leap for efficiency without convenience of retro-fitting or sacrificing convenience which would involve things like surface piercing propellers, kort nozzles, variable pitch propellers and something like 200 rpm for cruise speed.

Quote:

Originally Posted by Stumble

I'll try a different tack...

Let's assume you are ok motoring along at 3kn as your crusing speed in dead calm conditions. Which is about what you have, and eventually figure out that you can charge and use a battery bank at the same time, which in this application you should. Finally let's assume that we ignore battery efficency and Peukers law and all the other technical stuff for a few minutes....

Realistic goal to motor at 3 knots as well as charge the battery at the same time. I achieved 3 knots with 35% of the deck covered with solar, and had a lot of room for improvement.

Quote:

What happens when you drag anchor and need to reset in a 50kn headwind?

You should make sure your anchor holds in whatever wind is coming and/or seek adequate shelter. Most people don't know how to anchor. I did actually drag anchor in 50 knots because I didn't use enough scope. I didn't use a motor or engine and my boat is ok.

By this logic, anyone who cruises engine-free will be at danger of losing their boat. In reality, all the boats lost seem to have engines, and the engine-free sailors somehow manage to keep their boats. Maybe their engine happened to fail when they relied on it, where the engine free sailor was already in a safe place.

If you need to reset or are afraid of hitting rocks, you can sail upwind. It's proven that even with a huge engine, a small storm jib becomes more powerful in such conditions.

Yeah, really until either the proper tank testing or put the real boat in the water and test it happens, it is guesstimating , as I've said. It would be nice to know the margin of error plus or minus a couple of knots, so that's what I'm aiming for.

The new Cruise 4's have several improvements, although I think the prop was improved substantially quite awhile ago re reverse.

BTW, what do you think of using the Cruise 4 for the tender? The reason I raise it was I spoke to a guy cruising the GBR and parts north, and he said they went through so much more petrol for their 9,9 Yammie than they thought they would, in fact they ran out and had to head somewhere to re-fill the jerrycans. They were doing alot of diving and fishing everyday, so used the tender alot.

Was thinking the tender sunshade roof could be all semi rigid solar panels to keep the electric outboard battery topped up? You wouldn't go as fast as with a petrol outboard, but you'd keep going.

If you're travelling some distance, the best way to be efficient with a tender is to get it planing. I guess the cruise 4 could do that two up?